1. Field of the Invention
The present invention relates to pulse lasers such as an excimer laser, a halogen gas laser, a TEA-CO.sub.2 laser, TEMA-CO.sub.2 laser, and a metal vapor laser which excite a laser medium by pulse discharging and oscillate a laser beam by a resonator, and particularly to an X-ray preionization pulse laser having high laser oscillation efficiency.
2. Description of the Prior Art
Generally, the excimer laser, halogen gas laser, TEA-CO.sub.2 laser, TEMA-CO.sub.2 laser, metal vapor laser, etc., which excite a laser gas (a laser medium) to obtain laser oscillation employ a discharge excitation technique, except for large power lasers which employ an electron beam excitation technique.
FIG. 1 is a circuit diagram of a pulse laser employing the discharge excitation technique, according to the prior art.
This circuit has two main electrodes 103 and 104 disposed in a laser tube. The main electrodes 103 and 104 face each other and receive a pulse voltage to achieve pulse discharging for exciting a laser medium. To stabilize the laser medium excitation, it is necessary to preionize the laser medium. Usually, the preionization is carried out with ultraviolet rays generated by arc discharge from a plurality of spark gaps 112. The spark gaps 112 are driven by a pulse circuit that is different from a main electrode driving circuit 117.
The driving circuit 117 for the main electrodes 103 and 104 comprises a capacitor 101 (C.sub.1), a trigger gap (a switch) 105, and a charging resistor 113. Discharge from the main electrodes 103 and 104 is carried out several tens to hundreds of nanoseconds after discharge from the spark gaps 112.
FIG. 2 shows a conventional auto-preionization circuit for commonly causing the preionization discharge and main discharge. The operation of FIG. 2 will briefly be explained.
When a trigger gap (a switch) 105 is closed, a charge is transferred from a storage capacitor 101 (C.sub.1) to a capacitor 102 (C.sub.2) through preionization gaps 112, thereby charging the capacitor 102 (C.sub.2). After a laser medium is preionized by preionization from the preionization gaps 112 and after the capacitor 102 (C.sub.2) is sufficiently charged, main discharge between two main electrodes 103 and 104 is performed to oscillate a laser beam.
Generally, the capacitance of the capacitor C.sub.1 and that of the capacitor C.sub.2 are set to be C.sub.1 .gtoreq.C.sub.2.
To improve laser oscillation efficiency and stabilize discharging, it is necessary to sufficiently preionize the laser medium. The conventional technique involving ultraviolet rays generated by arc discharge from spark gaps is, however, incapable of sufficiently preionizing the laser medium, if the operating gas pressure for the main discharge is high or if the volume of the main discharge is too large. Namely, this conventional technique is not suitable for generating a strong laser beam. In addition, the speak gaps will be obstacles to gas flow, so that it may be difficult to operate at high repetition rates.
For large lasers, it is known to use an X-ray preionizes technique that preionize a laser medium with X-rays. The X-ray preionization technique can achieve sufficient preionization of the laser medium, even when the operating gas pressure for main discharge is high or even when the volume the main discharge is large. This technique employs, however, a separate power source for the preionization, in addition to a power source for the main discharge, thereby complicating the power source, enlarging the size and increasing the cost.
In contrast, the conventional preionization circuit utilizing ultraviolet rays generated by arc discharge from spark gaps cannot provide sufficient ultraviolet rays for preionizing a laser medium, thus demonstrating poor oscillation efficiency, unstable discharge, low laser output power, and low repetition rates. The conventional X-ray preionization circuit, on the other hand, has the problems of complicated power source and high cost.